Polycrystalline silicon ingot, preparation method thereof, and polycrystalline silicon wafer

ABSTRACT

Disclosed is a preparation method of a polycrystalline silicon ingot. The preparation method comprises: providing a silicon nucleation layer at the bottom of a crucible, and filling a silicon material above the silicon nucleation layer; heating the silicon material to melt same, adjusting the thermal field inside the crucible to make the melted silicon material to start crystallization on the basis of the silicon nucleation layer; and when the crystallization is finished, performing annealing and cooling to obtain a polycrystalline silicon ingot. By adopting the preparation method, a desirable initial nucleus can be obtained for a polycrystalline silicon ingot, so as to reduce dislocation multiplication during the growth of the polycrystalline silicon ingot. Further disclosed are a polycrystalline silicon ingot obtained through the preparation method and a polycrystalline silicon wafer made using the polycrystalline silicon ingot as a raw material.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Chinese Application No.201210096232.2, entitled A Polycrystalline Silicon Ingot, PreparationMethod Thereof, and Polycrystalline Silicon Wafer, filed on Apr. 1,2012, and Chinese Application No. 201210096188.5, entitledPolycrystalline Silicon Ingot, Preparation Method Thereof, andPolycrystalline Silicon Wafer, filed on Apr. 1, 2012, the entirecontents of which are hereby incorporated by reference. The presentapplication claims priority to Chinese Application No. 201210096291.X,entitled Polycrystalline Silicon Ingot and Preparation Method Thereof,Polycrystalline Silicon Wafer, and Crucible for Casting PolycrystallineSilicon Ingot, filed on Apr. 1, 2012, and Chinese Application No.201310033073.6, entitled Polycrystalline Silicon Ingot, PreparationMethod Thereof, and Polycrystalline Silicon Wafer, filed on Jan. 29,2013, the contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductor manufacture,particularly a polycrystalline silicon ingot, preparation methodthereof, and polycrystalline silicon wafer.

BACKGROUND OF THE INVENTION

As an emerging renewable green energy source, solar energy has become aresearch hotspot. With the rapid development of solar cell industry,polycrystalline silicon which is of low cost and suitable forlarge-scale production has become one of the leading photovoltaicmaterials in the art, and is gradually replacing the dominance of thetraditional silicon produced by Czochralski process in the solar cellmaterials market.

Currently, the preparation of polycrystalline silicon ingot mainlyadopts directional solidification system method (referred to as DSS) andcrystal growth furnace technique provided by GT Solar Company. Themethod generally comprises heating, melting, solidifying and growingcrystals, annealing and cooling steps, etc. In the solidificationprocess of crystal growth, accompanied by the continuous cooling of thebottom of the crucible, the melt silicon nucleated randomly andspontaneously, and then the random nucleation gradually grows. However,since the initial nucleation has not been controlled, the nucleationprocess is prone to generate dislocations, resulting in disorderedcrystalline orientations and nonuniform crystalline grains. Thus thepolycrystalline silicon ingot obtained by this method is of low quality.The photoelectric conversion efficiency of the solar cell produced bysuch polycrystalline silicon ingot is relatively low. Therefore, inorder to obtain a polycrystalline silicon ingot with low dislocationdensity, high-quality and less defects, it is very important to providea DSS method for efficiently creating good initial nucleation ofpolycrystalline silicon ingot.

SUMMARY OF THE INVENTION

In order to solve the above-mentioned technical problem, the presentinvention aims to provide a method for preparing polycrystalline siliconingot. The method enables to obtain good initial nucleation for thepolycrystalline silicon ingot, thus reduce the dislocations causedduring the growth of polycrystalline silicon ingot and obtainpolycrystalline silicon ingot of high quality. The present inventionalso provides polycrystalline silicon ingot of high quality obtained byusing such preparation method, and polycrystalline silicon waferobtained by using the polycrystalline silicon ingot as raw material.

In a first aspect, the present invention provides a method for preparingpolycrystalline silicon ingot, comprising:

providing a nucleating source of silicon material layer at the bottom ofa crucible, and feeding silicon onto the nucleating source of siliconmaterial layer;

melt the silicon by heating, and regulate thermal field in the crucibleto grow crystals from molten silicon on the base of the nucleatingsource of silicon material layer;

after the crystallization is completely finished, performing annealingand cooling to obtain polycrystalline silicon ingot.

The term “nucleating source of silicon material layer” herein refers toa nucleating source layer formed by silicon material. “Silicon” usedherein is a commonly encountered raw material used for casting ingot inthe art.

With regard to the method for preparing polycrystalline silicon ingotprovided by the present invention, the nucleation of molten silicon onthe solid silicon is homogeneous nucleation. Homogeneous nucleationrequires driving force much less than heterogeneous nucleation on quartzor ceramic crucible. Multiple even-distributed nucleating sources can beformed on the solid silicon, thus making polycrystalline silicon ingotto obtain good initial nucleation and growing crystals with dominantcrystal orientations. Further, solid silicon has an excellentheat-conducting property, making molten silicon to obtain more drivingforce during the nucleation, thus promoting control of the initialnucleation and growing crystals with dominant crystal orientations.

Preferably, the method for preparing polycrystalline silicon ingotcomprises:

coating inner wall of the crucible with a layer of silicon nitride,followed by feeding silicon from the bottom to the top of the crucible;

melting the silicon in the crucible to form molten silicon by heating;when solid-liquid interface formed by molten silicon and unmeltedsilicon is close to the bottom of the crucible, regulating thermal fieldto achieve supercooled state to grow crystals from molten silicon on thebase of the unmelted silicon;

after the crystallization of molten silicon is completely finished,performing annealing and cooling to obtain polycrystalline siliconingot;

the unmelted silicon is the nucleating source of silicon material layer.

Herein, a layer of silicon nitride provided on the inner wall ofcrucible can efficiently prevent impurities contained in the inner wallof crucible entering into crystals, and avoid “stick-pan” phenomenon ofpolycrystalline silicon ingot, thus improving quality of polycrystallinesilicon ingot and decreasing the operation difficulty of castingprocess.

When the silicon is not completely melted, the thermal field isregulated to achieve supercooled state to grow crystals from moltensilicon on the base of unmelted silicon.

Preferably, a layer of chunk heat conductor is laid between silicon andthe bottom of the crucible.

Preferably, the chunk heat conductor is chunk silicon or chunk graphite.

Preferably, the chunk silicon is one or more of chunk monocrystallinesilicon, chunk polycrystalline silicon and chunk noncrystalline silicon.

Preferably, thickness of the layer of chunk heat conductor is in a rangeof 1 cm-2 cm.

Chunk silicon and chunk graphite have excellent heat-conductingproperty, making molten silicon to obtain more driving force during thenucleation, thus promoting growth of crystalline grains with dominantcrystal orientations.

Preferably, during the melting process of silicon, the position ofsolid-liquid interface formed by molten silicon and unmelted silicon isdetected at 0.2-1 hour intervals.

Specifically, the position of solid-liquid interface formed by moltensilicon and unmelted silicon is detected by using quartz rod.

Preferably, in preliminary stage of melting process of silicon, theposition of solid-liquid interface formed by molten silicon and unmeltedsilicon is detected at 0.5-1 hour intervals.

Preferably, in later stage of melting process of silicon, the positionof solid-liquid interface formed by molten silicon and unmelted siliconis detected at 0.2-0.5 hour intervals.

When solid-liquid interface formed by molten silicon and unmeltedsilicon is detected to be close to the bottom of the crucible, thermalfield should be regulated to achieve supercooled state to grow crystalsfrom molten silicon on the base of the unmelted silicon.

Preferably, the step of regulating thermal field is regulating heatingpower to lower the temperature at a speed of 2-500 K/min.

Specifically, to reduce heating power of heating device or to switch offheating device, or to open heat radiator so as to achieve a supercooledstate in the thermal field where silicon ingot grows. Crystals nucleateand grow in such supercooled state.

Preferably, the method for preparing polycrystalline silicon ingotcomprises:

coating inner wall of the crucible with a layer of silicon nitride,followed by feeding silicon from the bottom to the top of the crucible;the step of feeding silicon further comprises laying a layer of crushedsilicon at the bottom of the crucible in advance, the crushed silicon isone or more of crushed monocrystalline silicon, crushed polycrystallinesilicon and non-crystalline silicon;

melting silicon in the crucible to form molten silicon by heating, whensolid-liquid interface formed by molten silicon and unmelted silicon isjust in or deep into the layer of crushed silicon, regulating thermalfield to achieve supercooled state to grow crystals from molten siliconon the base of the layer of crushed silicon;

after the crystallization of molten silicon is completely finished,performing annealing and cooling to obtain polycrystalline siliconingot.

The layer of crushed silicon is the nucleating source of siliconmaterial layer. Crushed silicon is laid at the bottom of the crucible inrandom order, and the layer forms a supporting structure having numerousholes. In melting process of silicon, molten silicon formed by meltingsilicon will fill holes. In preliminary stage of nucleation, under asupercooled state, multiple even-distributed nucleating sources form onthe layer of crushed silicon, thus making polycrystalline silicon ingotto obtain good initial nucleation and growing crystals with dominantcrystal orientations. Specifically, to control temperature to makemolten silicon which is just in solid-liquid interface formed by moltensilicon and unmelted silicon, and molten silicon which is filled in theholes to achieve supercooled state firstly, and nucleate and growcrystals. Consequently, interface of molten silicon moves away from thebottom of crucible, molten silicon grows crystals and solidifies.Initial nucleation of polycrystalline silicon ingot is controlled well,thus growing crystals with dominant beneficial crystal orientations, andavoiding large increase of dislocation to obtain polycrystalline siliconingot of high quality.

Preferably, size of the crushed silicon is in a range of 0.1 μm-10 cm;more preferably, size of the crushed silicon is in a range of 0.1 cm-10cm.

Herein, crushed silicon having a size of 0.1 82 m-10 μm is micro powder.

Preferably, thickness of the layer of crushed silicon is in a range of0.5 cm-5 cm.

It is not easy to lay a layer of crushed silicon which is too thin. Itis also hard to control. In addition, too thin the layer of crushedsilicon is bad for the formation of the complete supporting structure,as well as the subsequent nucleation.

Preferably, a layer of silicon nitride provided on the inner wall ofcrucible can efficiently prevent impurities contained in the inner wallof crucible entering into crystals, and avoid “stick-pan” phenomenon ofpolycrystalline silicon ingot, thus improving quality of polycrystallinesilicon ingot and decreasing the operation difficulty of castingprocess.

Preferably, during the melting process of silicon, the position ofsolid-liquid interface formed by melting silicon is detected at 0.2-1hour intervals.

Specifically, the position of solid-liquid interface formed by meltingsilicon is detected by using quartz rod.

Preferably, in preliminary stage of melting process of silicon, theposition of solid-liquid interface formed by melting silicon is detectedat 0.5-1 hour intervals.

Preferably, in later stage of melting process of silicon, the positionof solid-liquid interface formed by melting silicon is detected at0.2-0.5 hour intervals.

When solid-liquid interface formed by melting silicon is detected to bejust in or deep into the layer of crushed silicon, thermal field shouldbe regulated to achieve supercooled state to grow crystals from moltensilicon on the base of the layer of crushed silicon.

Preferably, the step of regulating thermal field is regulating heatingpower to lower the temperature at a speed of 2-500 K/min.

Specifically, to reduce heating power of heating device or to switch offheating device, or to open heat radiator so as to achieve a supercooledstate in the thermal field where silicon ingot grows. Nucleating andgrowing crystals in such supercooled state.

Preferably, the method for preparing polycrystalline silicon ingotcomprises:

(1) providing nucleating source at the bottom of the crucible to form anucleating source layer; the nucleating source is silicon powder;

(2) providing molten silicon above the nucleating source layer; the stepof providing molten silicon above the nucleating source layer is:feeding solid silicon onto the nucleating source layer, melting thesilicon by heating the crucible, so that the molten silicon is providedon the surface of the nucleating source layer; or heating solid siliconin another crucible to prepare molten silicon, followed by pouring themolten silicon into the crucible with nucleating source layer, so thatthe molten silicon is provided on the surface of the nucleating sourcelayer;

(3) controlling the temperature inside the crucible, the temperatureraising along the direction perpendicular to the bottom of the crucibleto form temperature gradient, making the molten silicon to nucleate andcrystallize by using the nucleating source, then obtainingpolycrystalline silicon ingot.

Preferably, the silicon powder can be provided at the bottom of thecrucible by applying or laying.

Preferably, particle size of silicon powder is in a range of 0.1 μm-1cm.

In step (3), the step of controlling thermal field in the crucible iscooling molten silicon to achieve supercooled state and nucleate andcrystallize. At that moment, presence of numerous silicon powder in goodfor molten silicon to nucleate rapidly.

Preferably, degree of supercooling is controlled to be in a range offrom −1K to −30K. Too low the degree of supercooling leads to slow heatdissipation, at that time (111) plane can develop sufficiently. However,too high the degree of supercooling leads to goo heat dissipation due tofast growth in the (110) (112) directions. A high degree of supercoolingis good for forming crystal orientations (110) (112) dominantly.Furthermore, because the grain boundary is area for atomic stagger,dislocations moving toward the grain boundary are absorbed. A suitableamount of the grain boundary can prevent dislocations increasing andexpanding, thus reducing the overall dislocations of silicon ingot andimproving the conversion efficiency of crystalline silicon.

Preferably, the method for preparing polycrystalline silicon ingotcomprises:

(1) laying a microcrystalline nucleating source layer at the bottom ofthe crucible, the microcrystalline nucleating source layer ismicrocrystalline silicon and/or amorphous silicon; thickness of themicrocrystalline nucleating source layer is a first height; themicrocrystalline nucleating source layer is the nucleating source ofsilicon material

(2) feeding silicon onto the microcrystalline nucleating source layer,melting the silicon to form molten silicon by heating, when solid-liquidinterface formed after the silicon is melted completely is just in ordeep into the microcrystalline nucleating source layer, regulatingthermal field to achieve supercooled state to grow crystals from themolten silicon on the base of the microcrystalline nucleating sourcelayer;

(3) after the crystallization of molten silicon is completely finished,performing annealing and cooling to obtain polycrystalline siliconingot.

Step (1) involves in providing a nucleating source of silicon materiallayer. Material of the nucleating source of silicon material layer whichprovides microcrystalline nucleation for the growth of silicon ingot ismicrocrystalline silicon and/or amorphous silicon.

The microcrystalline silicon, amorphous silicon is laid in any manner;there is no need to arrange manually. There are no limitations to thesize of the microcrystalline silicon and amorphous silicon. In addition,source and shape of microcrystalline silicon and amorphous silicon arenot limited. Purity of the microcrystalline silicon and amorphoussilicon should be no less than 3N.

Preferably, the microcrystalline silicon and/or amorphous silicon shouldbe in a form of rob, chunk, plate, strip or granule.

Preferably, the amorphous silicon is produced by methods such as Siemensmethod, improved Siemens method, and Fluidized-bed method.

Thickness of the nucleating source of silicon material layer is a firstheight, which can be determined as needed. Preferably, the first heightis in a range of 1-150 mm. More preferably, the first height is in arange of 5-150 mm. Further preferably, the first height is in a range of5-30 mm.

The crucible refers to container where polycrystalline silicon ingotgrows. The shape and type of the crucible are not limited.

Step (2) involves feeding silicon onto the nucleating source of siliconmaterial layer, melting the silicon to form molten silicon by heating,when solid-liquid interface formed after the silicon is meltedcompletely is just in or deep into the nucleating source of siliconmaterial layer, regulating thermal field to achieve supercooled state togrow crystals from the molten silicon on the base of the nucleatingsource of silicon material layer.

Preferably, in the case where the nucleating source of silicon materiallayer is microcrystalline silicon or amorphous silicon or combinationthereof, when solid-liquid interface formed after silicon is meltedcompletely is deep into the nucleating source of silicon material layerand apart from the bottom of the crucible at least 1 mm, thermal fieldis regulated to achieve supercooled state to grow crystals from themolten silicon on the base of microcrystalline silicon and/or amorphoussilicon.

More preferably, in the case where the nucleating source of siliconmaterial layer is microcrystalline silicon or amorphous silicon orcombination thereof, when solid-liquid interface formed after silicon ismelted completely is deep into the nucleating source of silicon materiallayer and apart from the bottom of the crucible at least 5 mm, thermalfield is regulated to achieve supercooled state to grow crystals fromthe molten silicon on the base of microcrystalline silicon and/oramorphous silicon.

The expression “just in the nucleating source of silicon material layer”herein means distance between the solid-liquid interface formed bymelting molten silicon and the bottom of the crucible equals to thefirst height.

Generally, silicon melts at 1500° C.-1560° C. Therefore, in the casewhere the nucleating source of silicon material layer ismicrocrystalline silicon or amorphous silicon, it will melt during theprocess of casting ingot. It is necessary to detect the position ofsolid-liquid interface formed by molten silicon. After silicon just meltcompletely or the nucleating source of silicon material layer meltpartially, start to regulate thermal field to nucleate and growcrystals.

Preferably, during the melting process of silicon, the position ofsolid-liquid interface formed after molten silicon is melted is detectedat 0.2-1 hour intervals.

Specifically, the position of solid-liquid interface formed after moltensilicon is melted is detected by using quartz rod.

Preferably, in preliminary stage of melting process of silicon, theposition of solid-liquid interface formed after molten silicon is meltedis detected at 0.5-1 hour intervals.

Preferably, in later stage of melting process of silicon, the positionof solid-liquid interface formed after molten silicon is melted isdetected at 0.2-0.5 hour intervals.

Preferably, the step of regulating thermal field is regulating heatingpower to lower the temperature at a speed of 2-30 K/min.

Specifically, to reduce heating power of heating device or to switch offheating device, or to open heat radiator so as to achieve a supercooledstate in the thermal field where silicon ingot grows. In the supercooledstate, crystals grow on the base of microcrystalline nucleation,controlling the temperature to rise along the direction perpendicular tothe bottom of the crucible to form temperature gradient.

Because microcrystalline material or amorphous material are arrayedregularly on a atomic or molecular basis, therefore such material insuch a range is equivalent to small microcrystalline crystals, which canbe used as microcrystalline nucleation for crystallization. When thesilicon melts, the molten silicon contacts the microcrystalline materialor amorphous material of the nucleating source layer. When thetemperature is further reduced, molten silicon grows on themicrocrystalline material or amorphous material. Since the presence ofnumerous micro crystals or microcrystalline nucleation similar to microcrystals contained in microcrystalline material or amorphous material,under the influence of such microcrystalline nucleation, numerous finecrystalline grains grow from molten silicon. After subsequent survivalof the fittest growth, crystals of fine uniform grain size and lowdislocation density is obtained.

Step (3) involves performing annealing and cooling to obtainpolycrystalline silicon ingot after the crystallization of moltensilicon is completely finished.

Since numerous fine crystalline grains grow from polycrystalline siliconingot by taking advantage of microcrystalline nucleation, such finecrystalline grains produce effect that similar to “necking down” toeliminate dislocations by grain boundary. Further, on the basis of thedominant crystalline orientation, after subsequent “survival of thefittest”, crystals with dominant beneficial crystal orientations grow,avoiding large increase of dislocation to obtain polycrystalline siliconingot of high quality.

The nucleating source of silicon material layer laid at the bottom ofcrucible provides fine nucleation points distributed uniformly, thusobtaining crystals of more fine uniform grain size, of a few defects andslow increase and expansion, and high photoelectric conversionefficiency.

Preferably, the method for preparing polycrystalline silicon ingotcomprises:

(1) randomly laying seed crystals with unlimited crystal orientation atthe bottom of crucible to form a layer of seed crystals, the layer ofseed crystals is the nucleating source of silicon material layer (2)providing molten silicon above the layer of seed crystals, controllingthe temperature at the bottom of the crucible below melting point of theseed crystals, making the layer of seed crystals not melt completely;

(3) controlling the temperature inside the crucible, the temperatureraising along the direction perpendicular to the bottom of the crucibleto form temperature gradient, making the molten silicon growing abovethe seed crystals, the molten silicon inheriting the structure of theseed crystals, then obtaining polycrystalline silicon ingot.

Seed crystals of step (1) should be laid in any manner. There is no needto arrange manually. There are no limitations to crystalline orientationof the seed crystals. In addition, source, type, shape, maximumside-length and dislocation density of the seed crystals are notlimited.

Preferably, seed crystals are materials from the top or the end ofsilicon ingot, materials from the edge of silicon ingot, defectivesilicon, crushed fragment of single crystals or finely crushed silicon.Materials from the top or the end of silicon ingot and materials fromthe edge of silicon ingot are commonly encountered waste materials.Defective silicon and crushed fragment of single crystals are producedduring sectioning process of crystalline silicon ingot. Finely crushedsilicon is produced by crushing crystalline waste materials of siliconingot.

Seed crystals should be single crystals or polycrystals. Molten siliconwill continue to grow above the seed crystals. The molten siliconinherits the structure of the seed crystals.

Seed crystals should be in a form of plate, chunk, strip or granule. Ifseed crystal is irregular in shape, crystalline orientation of seedcrystals distributes randomly, grain boundary is atomic disordered area.If seed crystal is regular in shape formed by cutting, due to thepolyhedron structure of crystals, laying randomly would cause disorderedcrystalline orientations. Grain boundary is also the area of atomicdisordered.

Preferably, maximum side-length of seed crystal is in a range of 1-100mm. The smaller the maximum side-length of seed crystal, the moredifferent the crystalline orientation of numerous seed crystals, it isprone to form grain boundary of the area for atomic stagger. Morepreferably, maximum side-length of seed crystal is in a range of 1-50mm.

Lower dislocation density of seed crystal is more conducive to growpolycrystalline silicon ingot of low dislocation density. Preferably,dislocation density of seed crystal is less than or equal to 10³(1/cm²).

Thickness of the layer of seed crystals is in a range of 0.5 cm-5 cm.Preferably, thickness of the layer of seed crystals is in a range of5-50 mm.

Thus, seed crystals are used as a nucleating source layer. Source ofseed crystals is very extensive and available easily. Compared to seedcrystals of continuous large size used in the art, production cost ofpolycrystalline silicon ingot is greatly reduced. In addition, seedcrystals are laid at the bottom of crucible randomly. There is no needto arrange manually, so it is easy to operate.

In step (2), manner of providing molten silicon above layer of seedcrystals is not limited. The step of providing molten silicon abovelayer of seed crystals should be: feeding solid silicon onto the layerof seed crystals, melting silicon by heating the crucible. At that time,the molten silicon is provided on the surface of the layer of seedcrystals. Preferably, providing molten silicon above layer of seedcrystals should be: heating solid silicon in another crucible to producemolten silicon, pouring the molten silicon into the crucible with thelayer of seed crystals. At that time, the molten silicon is provided onthe surface of the layer of seed crystals. Source and purity of solidsilicon are not limited.

The layer of seed crystals is not completely melted not completelymelted means the layer of seed crystals is melted partially and part ofthe layer of seed crystals maintain unmelted. Preferably, percentage ofunmelted seed crystals of initial seed crystals in step (1) is in arange of 5%-95%. Normally, silicon melts at a temperature in a range of1500° C.-1560° C. Temperature of layer of seed crystals at the bottom ofthe crucible is below melting point of seed crystals.

(3) controlling the temperature inside the crucible, the temperatureraising along the direction perpendicular to the bottom of the crucibleto form temperature gradient, making the molten silicon growing abovethe seed crystals, the molten silicon inheriting the structure of theseed crystals, then obtaining polycrystalline silicon ingot.

Seed crystals are laid at the bottom of crucible in any manner, and thecrystalline orientation is not limited, thus polycrystalline siliconingot of high quality can be obtained. It is because seed crystals laidin any manner provide suitable amount of grain boundary and the grainboundary is area for atomic stagger, dislocations moving toward thegrain boundary will be absorbed. Dislocations increasing and expandingcan be prevented, thus reducing the overall dislocations of siliconingot and improving the conversion efficiency and quality of crystallinesilicon.

In a second aspect, the present invention provides a polycrystallinesilicon ingot prepared by preparation methods as set forth above. Thepolycrystalline silicon ingot has low dislocation density and fewdefects.

In a third aspect, the present invention provides a polycrystallinesilicon wafer obtained by sectioning, slicing and cleaning thepolycrystalline silicon ingot as set forth above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the crucible after feeding feedstock,according to Example 1.

FIG. 2 is a map of minority carrier lifetime of the silicon ingot,according to Example 1.

FIG. 3 is a dislocation test pattern of the bottom of silicon ingot,according to Example 1.

FIG. 4 is a dislocation test pattern of the top of silicon ingot,according to Example 1.

FIG. 5 is a schematic view showing the crucible after feeding feedstock,according to Example 6.

FIG. 6 is a map of minority carrier lifetime of the polycrystallinesilicon ingot, according to Example 6.

FIG. 7 is a photoluminescence spectrum of polycrystalline silicon waferaccording to Example 6.

FIG. 8 is a schematic view showing the preparation according to Example9.

FIG. 9 is an image showing prevention of dislocation observed byphotoluminescence silicon wafer inspection system, according to Example9.

FIG. 10 is a map of minority carrier lifetime of the polycrystallinesilicon ingot, according to Example 9.

FIG. 11 is a map of minority carrier lifetime of the quasi-singlecrystal, according to comparative experiment 1.

FIG. 12 is a map of minority carrier lifetime of the polycrystallinesilicon ingot, according to comparative experiment 2.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

The invention will now be described in detail on the basis of preferredembodiments. It is to be understood that various changes may be madewithout departing from the spirit and scope of the inventions.

EXAMPLE 1

A method for preparing polycrystalline silicon ingot.

A layer of silicon nitride was provided on inner wall of a quartzcrucible by spray coating, followed by laying a layer of crushedpolycrystalline silicon having a size of 1-5 cm at the bottom of thecrucible. The layer of crushed polycrystalline silicon was 1 cm. Thenvarious chunk silicon was fed onto crushed polycrystalline silicon untilthe crucible was full. FIG. 1 is a schematic view of one embodiment,showing the crucible after feeding. 1 is crucible, 2 is crushedpolycrystalline silicon and 3 is silicon.

The crucible filled with silicon was then placed into a casting furnace.Starting a casting ingot program and vacuuming. And the temperature waselevated to the melting point of silicon by heating so as to melt thesilicon slowly. During the melting process, solid-liquid interfaceformed by molten silicon and unmelted silicon was detected by usingquartz rob. In preliminary stage of the melting process, the positionwas detected once every 2 hours (at 1 hour intervals). In later stage ofthe melting process, the position was detected at 0.5 hour intervals.

When solid-liquid interface formed by molten silicon and unmeltedsilicon was detected to be just in the layer of crushed polycrystallinesilicon, heat shield was open, thus reducing the temperature and coolingthe molten silicon. Temperature was reduced at a speed of 10 K/min. Asomewhat supercooled state was achieved. Molten silicon started to growcrystals on the base of unmelted silicon.

After the crystallization of molten silicon was completely finished,performing annealing and cooling to obtain polycrystalline siliconingot.

The polycrystalline silicon ingot as prepared was then cooled andsectioned to produce chunk polycrystalline silicon, followed by slicingand cleaning, polycrystalline silicon wafer was obtained. Thepolycrystalline silicon wafer was used as raw material to manufacturesolar cell by screen printing technique.

Minority carrier lifetime of the polycrystalline silicon ingot wastested using WT2000, the test results are shown in FIG. 2. It can beseen from the FIG. 2 that minority carrier lifetime of polycrystallinesilicon ingot distributes uniformly from the bottom (right) to the top(left), area for short minority carrier lifetime is small, silicon ingotis of good quality.

As for the obtained polycrystalline silicon ingot, dislocation wasobserved by using optical microscope (magnified 100 times). Test resultsshowed that: average dislocation density at the bottom of the siliconingot was 2.96×10⁴ (1/cm²), average dislocation density at the top ofthe silicon ingot was 3.41×10⁴ (1/cm²). FIG. 3 shows test results ofdislocation at the bottom of the silicon ingot. FIG. 4 shows testresults of dislocation at the top of the silicon ingot.

Photoelectric conversion efficiency of the solar cell was tested byusing a test system for solar cells (Halm, a German company). Testresults showed that the photoelectric conversion efficiency of the solarcell was 17.3%.

EXAMPLE 2

A method for preparing polycrystalline silicon ingot.

A layer of silicon nitride was provided on inner wall of a quartzcrucible by spray coating. And then a layer of chunk polycrystallinesilicon was laid at the bottom of the crucible, followed by laying alayer of crushed polycrystalline material having a size of 1-5 cm.Thickness of the layer of chunk polycrystalline silicon was 1 cm.Thickness of the layer of crushed polycrystalline material was 2 cm.After that, various chunk silicon was fed onto crushed polycrystallinematerial until the crucible was full.

The crucible filled with silicon was then placed into a casting furnace.Starting a casting ingot program and vacuuming. And the temperature waselevated to the melting point of silicon by heating so as to melt thesilicon slowly. During the melting process, solid-liquid interfaceformed by molten silicon and unmelted silicon was detected by usingquartz rob. In preliminary stage of the melting process, the positionwas detected once every 2 hours (at 1 hour intervals). In later stage ofthe melting process, the position was detected at 0.5 hour intervals.

When solid-liquid interface formed by molten silicon and unmeltedsilicon was detected to be deep into the layer of crushedpolycrystalline silicon a distance of 0.5 cm, heat shield was open, thusreducing the temperature and cooling the molten silicon. Temperature wasreduced at a speed of 20 K/min. A somewhat supercooled state wasachieved. Molten silicon started to grow crystals on the base ofunmelted silicon.

After the crystallization of molten silicon was completely finished,performing annealing and cooling to obtain polycrystalline siliconingot.

The polycrystalline silicon ingot as prepared was then cooled andsectioned to produce chunk polycrystalline silicon, followed by slicingand cleaning, polycrystalline silicon wafer was obtained. Thepolycrystalline silicon wafer was used as raw material to manufacturesolar cell by screen printing technique.

As for the obtained polycrystalline silicon ingot, dislocation wasobserved by using optical microscope (magnified 200 times). Test resultsshowed that: average dislocation density at the bottom of the siliconingot was 2.8×10⁴ (1/cm²), average dislocation density at the top of thesilicon ingot was 3.40×10⁴ (1/cm²).

Photoelectric conversion efficiency of the solar cell was tested byusing a test system for solar cells (Halm, a German company). Testresults showed that the photoelectric conversion efficiency of the solarcell was 17.46%.

EXAMPLE 3

A method for preparing polycrystalline silicon ingot.

A layer of silicon nitride was provided on inner wall of a quartzcrucible by spray coating. And then a layer of graphite plate of highstrength, high density and high purity was laid at the bottom of thecrucible, followed by laying a layer of crushed polycrystalline materialhaving a size of 1-5 cm. Thickness of the layer of graphite plate was 1cm. Thickness of the layer of crushed polycrystalline material was 0.5cm. After that, various chunk silicon was fed onto crushedpolycrystalline material until the crucible was full.

The crucible filled with silicon was then placed into a casting furnace.Starting a casting ingot program and vacuuming. And the temperature waselevated to the melting point of silicon by heating so as to melt thesilicon slowly. During the melting process, solid-liquid interfaceformed by molten silicon and unmelted silicon was detected by usingquartz rob. In preliminary stage of the melting process, the positionwas detected once every 2 hours (at 1 hour intervals). In later stage ofthe melting process, the position was detected at 0.5 hour intervals.

When solid-liquid interface formed by molten silicon and unmeltedsilicon was detected to be deep into the layer of crushedpolycrystalline silicon a distance of 0.2 cm, heat shield was open, thusreducing the temperature and cooling the molten silicon. Temperature wasreduced at a speed of 15 K/min. A somewhat supercooled state wasachieved. Molten silicon started to grow crystals on the base ofunmelted silicon.

After the crystallization of molten silicon was completely finished,performing annealing and cooling to obtain polycrystalline siliconingot.

The polycrystalline silicon ingot as prepared was then cooled andsectioned to produce chunk polycrystalline silicon, followed by slicingand cleaning, polycrystalline silicon wafer was obtained. Thepolycrystalline silicon wafer was used as raw material to manufacturesolar cell by screen printing technique.

As for the obtained polycrystalline silicon ingot, dislocation wasobserved by using optical microscope (magnified 200 times). Test resultsshowed that: average dislocation density at the bottom of the siliconingot was 3.1×10⁴ (1/cm²), average dislocation density at the top of thesilicon ingot was 3.56×10⁴ (1/cm²).

Photoelectric conversion efficiency of the solar cell was tested byusing a test system for solar cells (Halm, a German company). Testresults showed that the photoelectric conversion efficiency of the solarcell was 17.53%.

EXAMPLE 4

A method for preparing polycrystalline silicon ingot.

A layer of silicon nitride was provided on inner wall of a quartzcrucible by spray coating, followed by feeding various chunk silicononto the crucible from the bottom to the top until the crucible wasfull.

The crucible filled with silicon was then placed into a casting furnace.Starting a casting ingot program and vacuuming. And the temperature waselevated to the melting point of silicon by heating so as to melt thesilicon slowly. During the melting process, solid-liquid interfaceformed by molten silicon and unmelted silicon was detected by usingquartz rob. In preliminary stage of the melting process, the positionwas detected once every 2 hours (at 1 hour intervals). In later stage ofthe melting process, the position was detected at 0.5 hour intervals.

When the distance between the bottom of the crucible and thesolid-liquid interface formed by molten silicon and unmelted silicon wasdetected to be 0.2 cm, heat shield was open, thus reducing thetemperature and cooling the molten silicon. Temperature was reduced at aspeed of 15 K/min. A somewhat supercooled state was achieved. Moltensilicon started to grow crystals on the base of unmelted silicon.

After the crystallization of molten silicon was completely finished,performing annealing and cooling to obtain polycrystalline siliconingot.

The polycrystalline silicon ingot as prepared was then cooled andsectioned to produce chunk polycrystalline silicon, followed by slicingand cleaning, polycrystalline silicon wafer was obtained. Thepolycrystalline silicon wafer was used as raw material to manufacturesolar cell by screen printing technique.

As for the obtained polycrystalline silicon ingot, dislocation wasobserved by using optical microscope (magnified 200 times). Test resultsshowed that: average dislocation density at the bottom of the siliconingot was 3.12×10⁴ (1/cm²), average dislocation density at the top ofthe silicon ingot was 3.58×10⁴ (1/cm²).

Photoelectric conversion efficiency of the solar cell was tested byusing a test system for solar cells (Halm, a German company). Testresults showed that the photoelectric conversion efficiency of the solarcell was 17.48%.

EXAMPLE 5

A method for preparing polycrystalline silicon ingot.

(1) Nucleating source was provided at the bottom of a crucible to form anucleating source layer, wherein the step of providing nucleating sourceat the bottom of a crucible was: applying 200 g of silicon powder at thebottom of a crucible which had been coated with a layer of siliconnitride in advance. Then the crucible was roasted in an oven at 600° C.for 2 hours. Particle size of the silicon powder was 1 mm.

(2) Molten silicon was provided above the nucleating source layer,wherein the step of providing molten silicon above the nucleating sourcelayer was: feeding 450-800 kg of solid silicon above the nucleatingsource layer, then melting the solid silicon by heating the crucible andelevating the temperature to 1560° C. At that moment, molten silicon wasprovided on the surface of the nucleating source layer.

(3) controlling the temperature inside the crucible, the temperatureraising along the direction perpendicular to the bottom of the crucibleto form temperature gradient, making the molten silicon nucleating andforming crystals by using the nucleating source, then obtainingpolycrystalline silicon ingot. Herein, open heat shield and control thebottom temperature at 1360° C. to make the molten silicon liquid in asupercooled state, then nucleating and growing crystals by usingnucleating source. Polycrystalline silicon ingot was obtained.

Dislocation density of the polycrystalline silicon ingot preparedaccording to this embodiment was in a range of 3.6×10³-4.8×10³ (1/cm²),minority carrier lifetime was 18 microseconds (ms).

Polycrystalline silicon wafer prepared by using the polycrystallinesilicon ingot of this embodiment was suitable for manufacturing solarcell. Conversion efficiency of the obtained solar cell was 17.6%.

EXAMPLE 6

A method for preparing polycrystalline silicon ingot.

(1) Amorphous rob silicon of high purity produced by Siemens was laid atthe bottom of a crucible to form microcrystalline nucleating sourcelayer. Silicon was fed onto the microcrystalline nucleating source layeruntil the crucible was full. FIG. 5 is a schematic view of thisembodiment, showing the crucible after feeding. Thickness of themicrocrystalline nucleating source layer was 120 mm.

(2) The crucible filled with silicon was then placed into a castingfurnace. Starting a casting ingot program and vacuuming. And thetemperature was elevated to 1530° C. by heating so as to melt thesilicon slowly and form molten silicon. During the melting process,solid-liquid interface formed by molten silicon was detected by usingquartz rob. In preliminary stage of the melting process, the positionwas detected once every 2 hours (at 1 hour intervals). In later stage ofthe melting process, the position was detected at 0.5 hour intervals.

(3) When distance between the bottom of the crucible and thesolid-liquid interface formed by melting molten silicon was detected tobe 15 mm, heat shield was open, thus reducing the temperature andcooling the molten silicon. Temperature was reduced at a speed of 5K/min. A somewhat supercooled state was achieved. Molten silicon startedto grow crystals on the base of the amorphous rob silicon of highpurity.

(4) after the crystallization of molten silicon was completely finished,performing annealing and cooling to obtain polycrystalline siliconingot.

The polycrystalline silicon ingot as prepared was then cooled andsectioned to produce chunk polycrystalline silicon, followed by slicingand cleaning, polycrystalline silicon wafer was obtained. Thepolycrystalline silicon wafer was used as raw material to manufacturesolar cell by screen printing technique.

Minority carrier lifetime of the obtained polycrystalline silicon ingotwas tested by using WT2000. Test results were shown in FIG. 6. It can beseen from FIG. 6 that the polycrystalline silicon ingot was of longminority carrier lifetime and small dislocations.

As for the obtained polycrystalline silicon ingot, dislocation wasobserved by using optical microscope (magnified 200 times). Test resultsshowed that: average dislocation density at the bottom of the siliconingot was 2.2×10⁴ (1/cm²).

Dislocations of polycrystalline silicon wafer were inspected by usingphotoluminescence spectra. Test results were shown in FIG. 7. It can beseen from FIG. 7 that the polycrystalline silicon wafer was of smalldislocations and fine and uniform crystalline grains.

Photoelectric conversion efficiency of the solar cell was tested byusing a test system for solar cells (Halm, a German company). Testresults showed that the photoelectric conversion efficiency of the solarcell was 17.8%.

EXAMPLE 7

A method for preparing polycrystalline silicon ingot.

(1) Amorphous rob silicon of high purity produced by Siemens was crushedand then laid at the bottom of a crucible to form microcrystallinenucleating source layer. Silicon was fed onto the microcrystallinenucleating source layer. Thickness of the microcrystalline nucleatingsource layer was 50 mm.

(2) The crucible filled with silicon was then placed into a castingfurnace. Starting a casting ingot program and vacuuming. And thetemperature was elevated to 1540° C. by heating so as to melt thesilicon slowly and form molten silicon. During the melting process,solid-liquid interface formed by molten silicon was detected by usingquartz rob. In preliminary stage of the melting process, the positionwas detected once every 2 hours (at 1 hour intervals). In later stage ofthe melting process, the position was detected at 0.5 hour intervals.

(3) When distance between the bottom of the crucible and thesolid-liquid interface formed by molten silicon is detected to be 30 mm,heat shield was open, thus reducing the temperature and cooling themolten silicon. Temperature was reduced at a speed of 6 K/min. Asomewhat supercooled state was achieved. Molten silicon started to growcrystals on the base of the amorphous rob silicon of high purity.

(4) after the crystallization of molten silicon was completely finished,performing annealing and cooling to obtain polycrystalline siliconingot.

The polycrystalline silicon ingot as prepared was then cooled andsectioned to produce chunk polycrystalline silicon, followed by slicingand cleaning, polycrystalline silicon wafer was obtained. Thepolycrystalline silicon wafer was used as raw material to manufacturesolar cell by screen printing technique.

As for the obtained polycrystalline silicon ingot, dislocation wasobserved by using optical microscope (magnified 200 times). Test resultsshowed that: average dislocation density at the bottom of the siliconingot was 8.5×10³ (1/cm²).

Photoelectric conversion efficiency of the solar cell was tested byusing a test system for solar cells (Halm, a German company). Testresults showed that the photoelectric conversion efficiency of the solarcell was 18.0%.

EXAMPLE 8

A method for preparing polycrystalline silicon ingot.

(1) Silicon of high purity produced by Fluidized-bed method was laid atthe bottom of a crucible to form microcrystalline nucleating sourcelayer. Silicon was fed onto the microcrystalline nucleating source layeruntil the crucible was full. Thickness of the microcrystallinenucleating source layer was 15 mm.

(2) The crucible filled with silicon was then placed into a castingfurnace. Starting a casting ingot program and vacuuming. And thetemperature was elevated to 1500° C. by heating so as to melt thesilicon slowly. During the melting process, solid-liquid interfaceformed by molten silicon was detected by using quartz rob. Inpreliminary stage of the melting process, the position was detected onceevery 2 hours (at 1 hour intervals). In later stage of the meltingprocess, the position was detected at 0.5 hour intervals.

(3) When distance between the bottom of the crucible and thesolid-liquid interface formed by molten silicon was detected to be 10mm, heat shield was open, thus reducing the temperature and cooling themolten silicon. Temperature was reduced at a speed of 15 K/min. Asomewhat supercooled state was achieved. Molten silicon started to growcrystals on the base of the microcrystalline silicon.

(4) after the crystallization of molten silicon was completely finished,performing annealing and cooling to obtain polycrystalline siliconingot.

The polycrystalline silicon ingot as prepared was then cooled andsectioned to produce chunk polycrystalline silicon, followed by slicingand cleaning, polycrystalline silicon wafer was obtained. Thepolycrystalline silicon wafer was used as raw material to manufacturesolar cell by screen printing technique.

As for the obtained polycrystalline silicon ingot, dislocation wasobserved by using optical microscope (magnified 200 times). Test resultsshowed that: average dislocation density at the bottom of the siliconingot was 3.5×10⁴ (1/cm²).

Photoelectric conversion efficiency of the solar cell was tested byusing a test system for solar cells (Halm, a German company). Testresults showed that the photoelectric conversion efficiency of the solarcell was 17.6%.

EXAMPLE 9

A method for preparing polycrystalline silicon ingot.

(1) randomly laying seed crystals with unlimited crystal orientation atthe bottom of crucible to form a layer of seed crystals;

Seed crystals herein were pieces of single crystals produced in themanufacture of semiconductor. Seed crystals were single crystals in aform of plate. The maximum side-length was 20 mm, dislocation density isless than or equal to 10³ (1/cm²), thickness of the layer of seedcrystals was 50 mm.

(2) providing molten silicon above the layer of seed crystals,controlling the temperature at the bottom of the crucible below meltingpoint of the seed crystals, making the layer of seed crystals notcompletely melted not completely melted;

FIG. 8 shows the preparation according to this embodiment, wherein 1 iscrucible, 2 is layer of seed crystals, 3 is silicon. The step ofproviding molten silicon above the layer of seed crystals was: feedingsolid silicon, melting the silicon by heating the crucible and elevatingthe temperature to 1530° C. At that moment, molten silicon was providedon the surface of the layer of seed crystals. Temperature at the bottomof the crucible was 1412° C. Percentage of unmelted seed crystals ofinitial seed crystals in step (1) was 60%.

(3) controlling the temperature inside the crucible, the temperatureraising along the direction perpendicular to the bottom of the crucibleto form temperature gradient, making the molten silicon growing abovethe seed crystals, the molten silicon inheriting the structure of theseed crystals, then obtaining polycrystalline silicon ingot.

FIG. 9 is an image showing prevention of dislocation observed byphotoluminescence silicon wafer inspection system, according to thisembodiment. As shown in FIG. 9, 1 is grain boundary, 2 is no-dislocationarea, 3 is dislocation area, dislocations of grain boundary 1 moving isrefrained obviously, and no-dislocation area 2 and dislocation area 3formed at two sides of grain boundary 1.

Dislocation density of the polycrystalline silicon ingot preparedaccording to this embodiment was in a range of 1.5×10³-1.8×10³ (1/cm²),minority carrier lifetime was 25 microseconds (ms).

Polycrystalline silicon wafer prepared by using the polycrystallinesilicon ingot of this embodiment was suitable for manufacturing solarcell. Conversion efficiency of the obtained solar cell was 17.8%.

EXAMPLE 10

A method for preparing polycrystalline silicon ingot.

(1) randomly laying seed crystals with unlimited crystal orientation atthe bottom of crucible to form a layer of seed crystals;

Seed crystals herein were pieces of single crystals produced in themanufacture of semiconductor. Seed crystals were single crystals in aform of chunk. The maximum side-length was 100 mm, dislocation densitywas less than or equal to 10³ (1/cm²), thickness of the layer of seedcrystals was 50 mm.

(2) providing molten silicon above the layer of seed crystals,controlling the temperature at the bottom of the crucible below meltingpoint of the seed crystals, making the layer of seed crystals notcompletely melted not completely melted;

The step of providing molten silicon above the layer of seed crystalswas: feeding solid silicon, melting the silicon by heating the crucibleand elevating the temperature to 1560° C. At that moment, molten siliconwas provided on the surface of the layer of seed crystals. Temperatureat the bottom of the crucible was 1412° C. Percentage of unmelted seedcrystals of initial seed crystals in step (1) was 95%.

(3) controlling the temperature inside the crucible, the temperatureraising along the direction perpendicular to the bottom of the crucibleto form temperature gradient, making the molten silicon growing abovethe seed crystals, the molten silicon inheriting the structure of theseed crystals, then obtaining polycrystalline silicon ingot.

Dislocation density of the polycrystalline silicon ingot preparedaccording to this embodiment was in a range of 7.5×10³-8.0×10³ (1/cm²),minority carrier lifetime was 18 microseconds (ms).

Polycrystalline silicon wafer prepared by using the polycrystallinesilicon ingot of this embodiment was suitable for manufacturing solarcell. Conversion efficiency of the obtained solar cell was 17.8%.

EXAMPLE 11

A method for preparing polycrystalline silicon ingot.

(1) randomly laying seed crystals with unlimited crystal orientation atthe bottom of crucible to form a layer of seed crystals;

Seed crystals herein were pieces of single crystals produced in themanufacture of semiconductor. Seed crystals were single crystals in aform of granule. The maximum side-length was 1 mm, dislocation densitywas less than or equal to 10³ (1/cm²), thickness of the layer of seedcrystals was 5 mm.

(2) providing molten silicon above the layer of seed crystals,controlling the temperature at the bottom of the crucible below meltingpoint of the seed crystals, making the layer of seed crystals notcompletely melted not completely melted;

The step of providing molten silicon above the layer of seed crystalswas: feeding solid silicon, melting the silicon by heating the crucibleand elevating the temperature to 1500° C. At that moment, molten siliconwas provided on the surface of the layer of seed crystals. Temperatureat the bottom of the crucible was 1412° C. Percentage of unmelted seedcrystals of initial seed crystals in step (1) was 5%.

(3) controlling the temperature inside the crucible, the temperatureraising along the direction perpendicular to the bottom of the crucibleto form temperature gradient, making the molten silicon growing abovethe seed crystals, the molten silicon inheriting the structure of theseed crystals, then obtaining polycrystalline silicon ingot.

Dislocation density of the polycrystalline silicon ingot preparedaccording to this embodiment was in a range of 3.5×10⁴-4.8×10⁴ (1/cm²),minority carrier lifetime was 10 microseconds (ms).

Polycrystalline silicon wafer prepared by using the polycrystallinesilicon ingot of this embodiment was suitable for manufacturing solarcell. Conversion efficiency of the obtained solar cell was 17.1%.

EXAMPLE 12

A method for preparing polycrystalline silicon ingot.

(1) randomly laying seed crystals with unlimited crystal orientation atthe bottom of crucible to form a layer of seed crystals;

Seed crystals herein were pieces of single crystals produced in themanufacture of semiconductor. Seed crystals were single crystals in aform of granule. The maximum side-length was 50 mm, dislocation densitywas less than or equal to 10³ (1/cm²), thickness of the layer of seedcrystals was 50 mm.

(2) providing molten silicon above the layer of seed crystals,controlling the temperature at the bottom of the crucible below meltingpoint of the seed crystals, making the layer of seed crystals notcompletely melted;

The step of providing molten silicon above the layer of seed crystalswas: heating solid silicon in another crucible to prepare moltensilicon, followed by pouring the molten silicon into the crucible with alayer of seed crystals. At that moment, the molten silicon was providedon the surface of the layer of seed crystals. Temperature at the bottomof the crucible was 1413° C. Percentage of unmelted seed crystals ofinitial seed crystals in step (1) was 95%.

(3) controlling the temperature inside the crucible, the temperatureraising along the direction perpendicular to the bottom of the crucibleto form temperature gradient, making the molten silicon growing abovethe seed crystals, the molten silicon inheriting the structure of theseed crystals, then obtaining polycrystalline silicon ingot.

Dislocation density of the polycrystalline silicon ingot preparedaccording to this embodiment was in a range of 3.2×10⁴-3.8×10⁴ (1/cm²),minority carrier lifetime was 15 microseconds (ms).

Polycrystalline silicon wafer prepared by using the polycrystallinesilicon ingot of this embodiment was suitable for manufacturing solarcell. Conversion efficiency of the obtained solar cell was 17.5%.

EXAMPLE 13

A method for preparing polycrystalline silicon ingot.

(1) randomly laying seed crystals with unlimited crystal orientation atthe bottom of crucible to form a layer of seed crystals;

Seed crystals herein were pieces of single crystals produced in themanufacture of semiconductor. Seed crystals were polycrystals in a formof granule. The maximum side-length was 1 mm, dislocation density wasless than or equal to 10³ (1/cm²), thickness of the layer of seedcrystals was 5 mm.

(2) providing molten silicon above the layer of seed crystals,controlling the temperature at the bottom of the crucible below meltingpoint of the seed crystals, making the layer of seed crystals notcompletely melted;

The step of providing molten silicon above the layer of seed crystalswas: feeding solid silicon, melting the silicon by heating the crucibleand elevating the temperature to 1500° C. At that moment, molten siliconwas provided on the surface of the layer of seed crystals. Temperatureat the bottom of the crucible was 1412° C. Percentage of unmelted seedcrystals of initial seed crystals in step (1) was 60%.

(3) controlling the temperature inside the crucible, the temperatureraising along the direction perpendicular to the bottom of the crucibleto form temperature gradient, making the molten silicon growing abovethe seed crystals, the molten silicon inheriting the structure of theseed crystals, then obtaining polycrystalline silicon ingot.

Dislocation density of the polycrystalline silicon ingot preparedaccording to this embodiment was in a range of 1.2×10⁴-1.8×10⁴ (1/cm²),minority carrier lifetime was 10 microseconds (ms).

Polycrystalline silicon wafer prepared by using the polycrystallinesilicon ingot of this embodiment was suitable for manufacturing solarcell. Conversion efficiency of the obtained solar cell was 17.2%.

EXAMPLE 14

A method for preparing polycrystalline silicon ingot.

(1) randomly laying seed crystals with unlimited crystal orientation atthe bottom of crucible to form a layer of seed crystals;

Seed crystals herein were pieces of single crystals produced in themanufacture of semiconductor. Seed crystals were single crystals in aform of chunk. The maximum side-length was 40 mm, dislocation densitywas less than or equal to 10³ (1/cm²), thickness of the layer of seedcrystals was 40 mm.

(2) providing molten silicon above the layer of seed crystals,controlling the temperature at the bottom of the crucible below meltingpoint of the seed crystals, making the layer of seed crystals notcompletely melted;

The step of providing molten silicon above the layer of seed crystalswas: heating solid silicon in another crucible to prepare moltensilicon, followed by pouring the molten silicon into the crucible with alayer of seed crystals. At that moment, the molten silicon was providedon the surface of the layer of seed crystals. Temperature at the bottomof the crucible was 1413° C. Percentage of unmelted seed crystals ofinitial seed crystals in step (1) was 5%.

(3) controlling the temperature inside the crucible, the temperatureraising along the direction perpendicular to the bottom of the crucibleto form temperature gradient, making the molten silicon growing abovethe seed crystals, the molten silicon inheriting the structure of theseed crystals, then obtaining polycrystalline silicon ingot.

Dislocation density of the polycrystalline silicon ingot preparedaccording to this embodiment was in a range of 5.0×10³-5.6×10³ (1/cm²),minority carrier lifetime was 12 microseconds (ms).

Polycrystalline silicon wafer prepared by using the polycrystallinesilicon ingot of this embodiment was suitable for manufacturing solarcell. Conversion efficiency of the obtained solar cell was 17.4%.

EXAMPLE ILLUSTRATING THE EFFECTS

In order to support benefits of the present invention, comparativeexperiment data are provided below.

Comparative experiment 1: A complete single crystal rob was provided.After removing portions at the top or the end or the edge, it wassectioned into seed crystal cubes having a size of 156 mm×156 mm. Themonocrystalline cubes were laid at the bottom of crucible regularly,until the bottom of crucible was completely covered, followed by layingsilicon onto seed crystals. Melting at high temperature and controllingseed crystals at the bottom not completely melted.

Controlling temperature gradient and cooling the bottom first. Moltensilicon liquid grows crystals on the surface of seed crystals, andquasi-monocrystalline silicon ingot having a monocrystalline structurewas obtained.

Comparative experiment 2: Growth of an ordinary polycrystalline siliconingot comprising: feeding silicon into crucible, melting silicon byheating the crucible to control the thermal field in the crucible, so asto make molten silicon grows at the bottom of crucible and obtainpolycrystalline silicon ingot.

Comparison of Example 9, Example 10, comparative experiment 1 andcomparative experiment 2 are shown below:

TABLE 1 Comparison of Example 9, Example 10, comparative experiment 1and comparative experiment 2 Comparative Comparative Example 9 Example10 experiment 1 experiment 2 Characteristic In a form of Fragment fromthe Large area No of seed fragment edge of single crystals crystalsSource Waste material Crushed fragment Complete single No of seedproduced during from the edge of crystal rob obtained crystals themanufacture single crystals by sectioning after of semiconductorremoving portions at the top and the end and the edge Price No cost in 2RMB/kg High, 400-800 No non-silicon (non-silicon RMB/kg materialmaterial) (non-silicon material) Characteristic Multycrystals ofMultycrystals of Quasi-single Ordinary of product high efficiency highefficiency crystals multycrystals dislocation dislocation dislocationdislocation density lower density lower density lower density lower than10⁵ 1/cm², than 10⁵ 1/cm², than 10⁵ 1/cm², than 10⁵-10⁶ 1/cm², minoritycarrier minority carrier minority carrier minority carrier lifetime islifetime is lifetime is lifetime is 15~25 ms 10~20 ms 15~25 ms 5~10 ms

FIG. 10 is a map of minority carrier lifetime of the polycrystallinesilicon ingot, according to Example 9. FIG. 11 is a map of minoritycarrier lifetime of the quasi-single crystal, according to comparativeexperiment 1. FIG. 12 is a map of minority carrier lifetime of thepolycrystalline silicon ingot, according to comparative experiment 2. Itcan be seen from FIGS. 10-12 that, one embodiment of the presentinvention prepares polycrystalline silicon ingot of long minoritycarrier lifetime and few central area for minority carriers (area ofhigh dislocation in a certain extent). Comparative experiment 1 preparescentral area of quasi-single crystals exhibits a divergent pattern(indicating that dislocations are prone to expand). Comparativeexperiment 2 prepares polycrystalline silicon ingot of short minoritycarrier lifetime, large area for minority carrier having short lifetimein the center, and high dislocations.

Above all, layer of seed crystals of the present invention is nucleatingsource of silicon material layer. The obtained polycrystalline siliconingot having a dislocation density less than 10⁵ l/cm², minority carrierlifetime is in a range of 10-25 ms. However, silicon ingot obtained bytraditional method has a dislocation density in a range of 10⁵-10⁶l/cm², minority carrier lifetime is in a range of 5-10 ms. Thus thepolycrystalline silicon wafer prepared by using polycrystalline siliconingot is suitable for manufacturing solar cell. The prepared solar cellhas a conversion efficiency in a range of 17.1%˜17.8%, whereas solarcell prepared by using ordinary polycrystalline silicon wafer has aconversion efficiency in a range of 16.5˜16.9%. Efficiency ofquasi-single crystals is in a range of 17.2%-18.5%.

While the present invention has been described with reference toparticular embodiments, it will be understood that the embodiments areillustrative and that the invention scope is not so limited. Alternativeembodiments of the present invention will become apparent to thosehaving ordinary skill in the art to which the present inventionpertains. Such alternate embodiments are considered to be encompassedwithin the spirit and scope of the present invention. Accordingly, thescope of the present invention is described by the appended claims andis supported by the foregoing description.

What is claimed is:
 1. A method for preparing polycrystalline siliconingot, comprising: (1) laying a microcrystalline nucleating source layerat the bottom of the crucible, the microcrystalline nucleating sourcelayer is microcrystalline silicon and/or amorphous silicon; thickness ofthe microcrystalline nucleating source layer is a first height; themicrocrystalline nucleating source layer is the nucleating source ofsilicon material layer; (2) feeding silicon onto the microcrystallinenucleating source layer, melting the silicon to form molten silicon byheating, wherein a solid-liquid interface formed after the silicon ismelted completely reaches the surface of the microcrystalline nucleatingsource layer or is deep into the microcrystalline nucleating sourcelayer and the height to the bottom of the crucible is greater than orequal to 1 mm, regulating a thermal field to achieve supercooled stateto grow crystals from the molten silicon on the base of themicrocrystalline nucleating source layer; (3) after the crystallizationof molten silicon is completely finished, performing annealing andcooling to obtain polycrystalline silicon ingot.
 2. The method forpreparing polycrystalline silicon ingot according to claim 1, whensolid-liquid interface formed after the silicon is melted completely isdeep into the microcrystalline nucleating source layer and the height tothe bottom of the crucible is greater than or equal to 5 mm, regulatingthe thermal field to achieve supercooled state to grow crystals from themolten silicon on the base of the microcrystalline nucleating sourcelayer.
 3. The method for preparing polycrystalline silicon ingotaccording to claim 1, wherein the first height is in a range of 1-150mm.